In sweetpotato (Ipomoea batatas), the successful emergence and development of lateral roots (LRs), the main determinant of root system architecture (RSA), determines the competency of adventitious roots (ARs) to undergo storage root formation. The present study investigated the effect of three levels of root-knot nematode (RKN) inoculum of race 3 of Meloidogyne incognita on LR length, number, area, and volume in ‘Beauregard’, ‘Evangeline’, and ‘Bayou Belle’, sweetpotato cultivars which are highly susceptible, moderately resistant, and highly resistant, respectively, to M. incognita. The three RKN levels were control (untreated), medium (500 eggs/pot), and high (5000 eggs/pot). In general, the number of galls after 20 days for each cultivar was consistent across RKN levels and two planting dates (PDs). ‘Beauregard’ inoculated with medium and high RKN levels showed 2.9 and 18.9 galls on each AR, respectively. ‘Evangeline’ had 0.5 and 3.4 galls at medium and high RKN levels, respectively. By contrast, ‘Bayou Belle’ showed only 0.9 galls at the high inoculum level. There was a significant PD × cultivar effect and cultivar × RKN level effect for all root attributes. LR attributes varied within and among resistant and susceptible cultivars with a general trend for increase in all root growth attributes in response to RKN infection in the first (PD1) and second PD (PD2). ‘Evangeline’ showed relatively consistent within-cultivar increase across PD1 (medium and high RKN levels) and PD2 (medium RKN level only). LR length, number, area, and volume within ‘Evangeline’ plants subjected to high RKN increased 122%, 126%, 154%, and 136%, respectively, relative to the untreated control plants in PD1. ‘Evangeline’ (PD1 and PD2) and ‘Bayou Belle’ (PD1 only) showed significant increase in all root attributes relative to the susceptible ‘Beauregard’ at medium or high RKN levels. In PD1, LR length, number, area, and volume in ‘Evangeline’ plants subjected to high RKN increased 165%, 167%, 176%, and 190%, respectively, relative to ‘Beauregard’ plants at the same RKN level. These findings are consistent with some data in other systems wherein nematode infection is associated with cultivar-specific root compensatory growth and demonstrate how genotype and environment interact to modify root development responses. These data can be used to further understand the role of cultivar-specific responses to nematode infection and can lead to the consideration of root traits in selection strategies.
Douglas Miano, Don LaBonte and Christopher Clark
Sweetpotato is an important staple food crop in Sub-Saharan Africa, with production being concentrated in East Africa, particularly around Lake Victoria. Productivity of the crop is greatly constrained by viral diseases. Four main viruses have consistently been detected from various surveys done in the region viz., sweetpotato feathery mottle virus (SPFMV), sweetpotato chlorotic stunt virus (SPCSV), sweetpotato mild mottle virus (Sp.m.MV), and sweetpotato chlorotic fleck virus (SPCFV). The most severe symptoms have been caused by co-infection with SPCSV and SPFMV, resulting in the synergistic sweetpotato virus disease (SPVD). Some local sweetpotato genotypes have been reported to recover from, or have localized distribution of SPVD, suggesting that the disease is not fully systemic. This has led to the suggestion that uninfected cuttings may be obtained from previously infected plants. Experiments were set to determine the possibility of obtaining cuttings long enough for propagation that are free from virus infection. This would form a basis for recommending to the local small-holder farmers of a way to reduce losses due to the disease. Field-grown sweetpotato vines were cut into three pieces (15, 15–30, and >30 cm from the apex) and tested for SPCSV and SPFMV. Nine genotypes were selected from a group of 21 local clones and used for this study. The two viruses were equally present in all the three sections of infected vines, indicating that it is not easy to obtain a virus-free cutting for field propagation from an infected vine. Virus assays in the past has mainly been limited to the use of serological methods. Use of PCR resulted in detection of begomoviruses infecting sweetpotatoes for the first time in the region.
Blair Buckley III and Christopher A. Clark
Bacterial blight, incited by Xanthomonas axonopodis pv. vignicola, is a major disease of cowpea. Strong resistance has not been reported in commercial cultivars. Many cultivars released within the last ten years have not been previously screened. Thirty-eight cowpea cultivars, 60 breeding lines, and 25 PIs were screened for reaction to bacterial blight in tests conducted in a greenhouse. Inoculum was adjusted turbidimetrically (OD620nm = 0.1) to approximately 108 cfu/ml. Seedling plants were inoculated by either leaf infiltration or stem puncture methods. Ratings were on a 1-6 scale in which 1 = no symptoms, 2 = localized lesions at site of inoculation, 3 = lesions spreading slightly near site of inoculation, 4 = any symptoms on systemically infected leaves or stems, 5 = extensive wilting and/or stem collapse, 6 = dead plant. All cultivars were susceptible to the pathogen. Cream-type cultivars Bettergreen, Tender Cream, Zipper Cream, Carolina Cream, and Mississippi Cream were among the most susceptible. Breeding lines MN13, MN150, TX57069-11, TX 58048-2000, and TX 59069-11 produced hypersensitive reactions in response to leaf infiltration inoculation. However, the three TX lines were rated susceptible when inoculated by stem puncture. Eighteen PIs (including PI293467, PI293521, PI293525, PI293567, and PI293571) were highly resistant to bacterial blight.
Christopher J. Clark and Douglas M. Burmeister
Development of browning induced in `Braeburn' apple (Malus ×domestica Borkh.) fruit by a damaging CO2 concentration was monitored weekly using magnetic resonance imaging (MRI) during a 4-week storage trial (0.5 °C, 2 kPa O2/7 kPa CO2). Discrete patches of high-intensity signal, distributed randomly throughout the fruit, were observed in multislice images of samples after 2 weeks of storage; these patches were eventually confirmed as being sites of browning reactions after dissection at the end of the trial. Subsequently (weeks 3 and 4), signal intensity at sites of incipient damage increased and patches enlarged and coalesced. After 2 weeks of storage, the extent of affected tissue, averaged across all image slices, was 1.5%, increasing to 15.9% and 21.3% after 3 and 4 weeks. The average rate at which tissue damage spread in individual slices was 0.81 (range: 0–3.70) cm2·d–1 between weeks 2 and 3, declining to 0.32 (range: 0–1.55) cm2·d–1 in the final week. Tissue damage induced under these conditions did not spread at the same rate at all locations within individual fruit, nor was it preferentially located toward the stem or calyx ends of the fruit.
Frank A. Buffone, Don R. La Bonte and Christopher A. Clark
DNA isolated from Fusarium lateritium Nees: Fr.-infected `Jewel' sweetpotato [Ipomoea batatas (L.) Lam.] plants was compared to F. lateritium-free `Jewel' plants for differences in random amplified polymorphic DNA (RAPD) marker products. Differences in RAPD marker products were detected. Amplified DNA isolations from F. lateritium-infected `Jewel' plants generated additional, unique DNA fragments not found in amplified DNA isolations of F. lateritium-free `Jewel' plants. These unique amplified DNA fragments were consistent with those obtained from amplified DNA isolations of the F. lateritium isolate, 91-27-2, used for inoculation. We found that F. lateritium DNA successfully competes with sweetpotato DNA in the polymerase chain reaction for priming sites in a 3: 1 ratio of sweetpotato DNA to F. lateritium DNA. Our results indicate the importance of avoiding plant material infested with pathogens to avoid spurious marker bands.
Arthur Villordon, Christopher Clark, Tara Smith, Don Ferrin and Don LaBonte
Forward and stepwise regression methods identified variables related to the influence of transplanting date on yield of U.S. #1 sweetpotatoes. The variables were mean minimum soil temperature 5 days after transplanting (DAT), wind direction at transplanting, and accumulated heat units (growing degree-days) 5 DAT. Machine learning techniques identified the same variables using leave-one-out and k-fold cross-validation methods. Growers and crop consultants, in collaboration with knowledge workers, can use this information in conjunction with public and subscription-based weather forecasts to further optimize transplanting date determination and for making risk-averse decisions. These results help to underscore the importance of consistent transplant establishment as one of the determinants of storage root yield in sweetpotatoes.
Arthur Villordon, Julio Solis, Don LaBonte and Christopher Clark
A prototype phenology-driven Bayesian belief network (BBN) model, named BxNET, was developed to represent the relationship between fresh market yield (U.S. #1 grade) and agroclimatic variables known to influence the critical storage root initiation stages in ‘Beauregard’ sweetpotato. This data-driven model was developed from experimental data collected over 3 years of field trials in which management variables were kept as uniform as possible. The BBN was developed assuming that soil moisture measured at the 15-cm depth was not a limiting variable during the first 20 days after transplanting, during which the onset of storage root initiation determined the majority of storage root yield at harvest. The absence of influence from weeds, disease, insect pests, and chemical injury was also assumed. Accuracy of the fully parameterized working prototype was estimated through leave-one-out cross-validation (14% error rate), validation on an independent test data set (20% error rate), and area under the receiving operator characteristic curve (0.59) analysis. As a result of its empirical nature, BxNET is only applicable to the cultivar, location, and the limited set of environmental (air temperature, soil temperature, relative humidity, solar radiation) and management variables as defined in the 3-year study. This beta-level model can serve as a foundation for the development of a final working model through further testing and validation. Additional validation data may require revision of the current model structure and conditional probabilities. These validation studies will also allow the model to be used in other locations. BxNET can be expanded to include other causal variables such as weed incidence, disease presence, insects, and chemical injury. Such an expansion can lead to the development of a model-based decision support system for sweetpotato production. Such a system can help model alternative management scenarios and determine the most reasonable management interventions to achieve optimum yield outcomes under different agroclimatic conditions.
Arthur Villordon, Christopher Clark, Don LaBonte and Nurit Firon
This study aimed to investigate the effect of 1-methylcyclopropene (1-MCP) on adventitious rooting in two sweetpotato cultivars. Experiments with ‘Beauregard’ and ‘Evangeline’ sweetpotato cuttings revealed differential adventitious root (AR) emergence responses to 1-MCP application. ‘Beauregard’ AR count and length decreased with 1-MCP application in two of four experiments. In contrast, 1-MCP did not influence ‘Evangeline’ root count. However, ‘Evangeline’ root length decreased in three of four experiments. Trypan blue staining of ‘Beauregard’ nodal tissue with delayed AR primordia emergence showed localized dead tissue in the general area where ARs emerge. The degree of staining appeared to correspond with the stage of AR emergence with the staining becoming more intense around the time an AR primordium eventually emerged through a crack in the epidermis. This response agrees with reported results of ethylene-mediated AR emergence in other plant species. These results also appear to suggest that ‘Beauregard’ and ‘Evangeline’ cuttings differ in ethylene sensitivity. This represents the first evidence of genotype-specific ethylene involvement in adventitious rooting of sweetpotato cuttings.
Heather L. Wallace, Don R. La Bonte and Christopher A. Clark
Virus infections and genetic mutations have been implicated in the decline of sweetpotato yield and quality. Virus-tested mericlones were derived from 12 infected clones of `Beauregard' sweetpotato by meristem-tip culture. Field studies were conducted to evaluate yield differences between the virus-tested and the virus-infected plants of each respective clone. After a 90-day growing period, the storage roots were harvested, weighed, and analyzed with a colorimeter to gauge color of skin and flesh. Yield was 7% to 130% greater in virus-tested mericlones compared to their respective virus-infected clone. Data also show these 12 virus-tested mericlones vary in yield by up to 118%. This suggests genetic differences between clones greatly affect yield. The virus-tested mericlones also show a more desirable darker-red hue for skin and flesh than the virus-infected clones. The incorporation of virus-tested material into foundation seed programs could potentially increase yield and quality with little added expense to growers, thereby netting a higher return on their crop.
Mary Helen Ferguson, Christopher A. Clark and Barbara J. Smith
Xylella fastidiosa Wells et al. causes disease in a number of plants in the southeastern United States, including southern highbush blueberry (Vaccinium corymbosum interspecific hybrids), but little was known concerning its potential impact in rabbiteye blueberry (Vaccinium virgatum Aiton syn. Vaccinium ashei Reade). In a naturally infected orchard in Louisiana, mean yields of X. fastidiosa–positive plants were 55% and 62% less than those of X. fastidiosa–negative plants in 2013 and 2014, respectively. Average berry weight was also lower in X. fastidiosa–positive plants. Within 3 years of testing positive for X. fastidiosa, four of nine X. fastidiosa–positive plants appeared dead. However, plants that were X. fastidiosa–negative in 2013 remained so until 2015, indicating that the bacterium did not spread rapidly in this established orchard during this time. Other factors, including soil chemistry variables, Phytophthora cinnamomi, ring nematode, and ringspot symptoms, were also investigated to determine if one of these might predispose plants to infection with X. fastidiosa or be partly responsible for observed yield loss. In most cases, interactions were not found, but associations with soil Cu and Zn suggest a need for further research on whether these elements predispose rabbiteye blueberry to X. fastidiosa infection and thereby contribute to yield losses. Researchers, extension workers, and growers should be aware of X. fastidiosa as a potential yield- and survival-impacting factor in rabbiteye blueberry.